Prior art fully shielded signal lines for high-frequency circuits include coaxial cables with a rectangular cross-section and fully shielded "triplate" cables such as are used to connect high- and ultra-high-frequency circuits, specially in integrated microwave circuits.
A prior art process for fabrication of fully shielded signal lines in semiconductor-high-frequency circuits is disclosed in Proceedings of Electronic Components Conference, Washington, 1985, pp. 384-388, 386. That process includes, along with other process steps, (a) the application of a photolithographic film onto a conductive substrate, (b) forming gaps in the photosensitive film corresponding to portions of the outer conductive walls of a rectangular coaxial cable, and (c) electrodepositing copper in those gaps to form corresponding portions of the walls. In that process, after step (c), heat and pressure is used to laminate an insulating prepreg film onto both the exposed portions of the substrate and onto the conductive tracks; the insulating material covering the conductive tracks is then selectively mechanically removed in a subsequent surfacing step, whereupon a similar layer of copper tracks separated by an insulating material may be formed on the first layer, with at least some of the some conductive tracks of the first layer in electrical contact with corresponding conductive tracks in the second layer. The required mechanical removal of the prepreg material is however difficult to perform and is expensive.